There's no new copyright, the clock starts ticking when the author dies. There is always, in principal, an heir. Everyone has relatives, even if they are distant. The person with a claim to the estate may not even be aware that they have a claim. So the book can sit in limbo with no one legally able to copy it because it is impossible to obtain permission. Physical property probablt isn't going to be allowed to sit abandoned for decade after decade.
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